Eleven normotensive pregnant subjects (29+/-1 years; gestational age, 39+/-2 weeks; blood pressure 114+/-2 / 74+/-2 mmHg) had plasma levels of marinobufagenin increased three-fold (1.38 +/-0.40 vs. 0.38 +/- 0.10 nmol/L; P<0.01) and lower activity of Na/K-ATPase in erythrocytes (1.16 +/- 0.11 vs. 2.80 +/- 0.20 micromol Pi/ml/hr; P<0.01) vs. 12 patients with PE (30 +/- 1 years; gestational age, 37.9 +/- 0.3 weeks; blood pressure 159 +/- 5 / 100 +/- 3 mmHg). Ex vivo, DigiFab (1 microgramm/mL) restored erythrocyte Na/K-ATPase activity (1.72 +/- 0.13 micromol Pi/ml/hr; P<0.01) and 3 mmol magnesium sulfate potentiated the effect of DigiFab (2.30 +/- 0.20 micromol Pi/ml/hr; P<0.01). The main observation of this study is that MgSO4 potentiates the effect of DigiFab with respect to reversal of preeclampsia-induced inhibition of Na/K-ATPase by an endogenous MBG. In preeclampsia, elevated plasma levels of MBG exhibit pro-hypertensive effect via inhibition of vascular Na/K-ATPase. Erythrocyte Na/K-ATPase is sensitive to endogenous CTS, and changes in its activity could serve as a marker characterizing the magnitude of CTS-dependent effects. Accordingly, in the present study heightened levels of MBG in patients with preeclampsia were associated with substantial inhibition of erythrocyte Na/K-ATPase, while ex vivo incubation of blood from preeclamptic patients in the presence of the CTS-neutralizing antibody, DigiFab, restored Na/K-ATPase activity. In the present study DigiFab ex vivo restored Na/K-ATPase activity, and an increase in levels of Mg2+ ions in incubation medium potentiated the effect of DigiFab. Accordingly, in vitro, MBG potently inhibited Na/K-ATPase in erythrocytes from control subjects and Mg2+ desensitized Na/K-ATPase to MBG. These findings agree with previous clinical data demonstrating that MgSO4 is capable of offsetting the deleterious cardiac effects of digitalis overdose, and that Mg2+ ions are capable of changing Na/K-ATPase conformation. Our present observations suggest that MgSO4 is capable of increasing the efficacy of DigiFab-mediated immunoneutralization of MBG-induced Na/K-ATPase inhibition in pathological states in which levels of CTS are elevated.